Use of thinnings and other low specific gravity wood for lyocell products method

ABSTRACT

The use of low specific gravity wood from thinning operations, for example, will produce a lower brownstock viscosity for a given kappa number target. A differential of 200-cP falling ball pulp viscosity has been detected from Kraft cooks of low and high specific gravity wood. Using low specific gravity wood can reduce the bleach stage temperature and the chemical dose needed in the bleach plant to produce lyocell pulp specifications. Low specific gravity wood also increases the ability to reduce pulp viscosity to very low levels without increasing the copper number of the pulp or the concentration of carbonyl in the pulp above acceptable levels.

CROSS-REFERENCES TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of U.S. patentapplication Ser. No. 09/842,274 filed Apr. 24, 2001, which in turn is acontinuation-in-part of U.S. patent application Ser. No. 09/574,538,filed May 18, 2000, now U.S. Pat. No. 6,331,354, which is acontinuation-in-part of U.S. patent application Ser. No. 09/256,197,filed Feb. 24, 1999, now U.S. Pat. No. 6,210,801. All the aboveapplications are herein fully incorporated by reference.

FIELD OF THE INVENTION

[0002] The present invention is directed to pulps useful for makinglyocell-molded bodies, including films, fibers, and non-woven webs, andto methods of making such pulps useful for making the lyocell-moldedbodies, to the lyocell-molded bodies made from the pulps and to themethods for making the lyocell-molded bodies. In particular, the presentinvention is directed to using “young” wood (often characterized as“core wood”, “juvenile wood”, “low specific gravity wood” or, in somecases as “thinnings”.).

BACKGROUND OF THE INVENTION

[0003] Cellulose is a polymer of D-glucose and is a structural componentof plant cell walls. These cells are referred to as fibers. Cellulosicfibers are especially abundant in tree trunks from which they areextracted, converted into pulp, and thereafter utilized to manufacture avariety of products.

[0004] Rayon is the name given to a fibrous form of regeneratedcellulose that is extensively used in the textile industry tomanufacture articles of clothing. For over a century, strong fibers ofrayon have been produced by the viscose and cuprammonium processes. Thelatter process was first patented in 1890 and the viscose process twoyears later. In the viscose process cellulose is first steeped in amercerizing strength caustic soda solution to form an alkali cellulose.The cellulose is then reacted with carbon disulfide to form cellulosexanthate, which is then dissolved in dilute caustic soda solution. Afterfiltration and deaeration, the xanthate solution is extruded fromsubmerged spinnerets into a regenerating bath of sulfuric acid, sodiumsulfate, and zinc sulfate to form continuous filaments. The resultingviscose rayon is presently used in textiles and was formerly widely usedfor reinforcing rubber articles such as tires and drive belts.

[0005] Cellulose is also soluble in a solution of ammonia copper oxide.This property forms the basis for production of cuprammonium rayon. Thecellulose solution is forced through submerged spinnerets into asolution of 5% caustic soda or dilute sulfuric acid to form the fibers,which are then decoppered and washed. Cuprammonium rayon is available infibers of very low deniers and is used almost exclusively in textiles.

[0006] The foregoing processes for preparing rayon both require that thecellulose be chemically derivatized or complexed in order to render itsoluble and therefore capable of being spun into fibers. In the viscoseprocess, the cellulose is derivatized, while in the cuprammonium rayonprocess, the cellulose is complexed. In either process, the derivatizedor complexed cellulose must be regenerated and the reagents used tosolubilize it must be removed. The derivatization and regeneration stepsin the production of rayon significantly add to the cost of this form ofcellulose fiber. Consequently, in recent years attempts have been madeto identify solvents that are capable of dissolving underivatizedcellulose to form a dope of underivatized cellulose from which fiberscan be spun.

[0007] One class of organic solvents useful for dissolving cellulose arethe amine N-oxides, in particular the tertiary amine N-oxides. Forexample, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group ofamine oxide materials suitable as solvents. Johnson, in U.S. Pat. No.3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide(NMMO) and other amine N-oxides as solvents for cellulose and many othernatural and synthetic polymers. Franks et al., in U.S. Pat. Nos.4,145,532 and 4,196,282, deal with the difficulties of dissolvingcellulose in amine oxide solvents and of achieving higher concentrationsof cellulose.

[0008] Lyocell is an accepted generic term for a cellulose fiberprecipitated from an organic solution in which no substitution ofhydroxyl groups takes place and no chemical intermediates are formed.Several manufacturers presently produce lyocell fibers, principally foruse in the textile industry. For example, Acordis, Ltd. presentlymanufactures and sells a lyocell fiber called Tencel® fiber.

[0009] Currently available lyocell fibers are produced from wood pulpsthat have been extensively processed to remove non-cellulose components,especially hemicellulose. These highly processed pulps are referred toas dissolving grade or high alpha (or high α) pulps, where the termalpha (or α) refers to the percentage of cellulose. Thus, a high alphapulp contains a high percentage of cellulose, and a correspondingly lowpercentage of other components, especially hemicellulose. The processingrequired to generate a high alpha pulp significantly adds to the cost oflyocell fibers and products manufactured therefrom.

[0010] Since the conventional Kraft process stabilizes residualhemicelluloses against further alkaline attack, it is not possible toobtain acceptable high alpha pulps for lyocell products, throughsubsequent treatment of Kraft pulp in the conventional bleaching stages.In order to prepare high alpha pulps by the Kraft process, it isnecessary to pretreat the wood chips in an acid phase before thealkaline pulping stage. A significant amount of material, primarilyhemicellulose, on the order of 10% or greater of the original woodsubstance, is solubilized in this acid phase pretreatment and thusprocess yields drop. Under these conditions, the cellulose is largelyresistant to attack, but the residual hemicelluloses are degraded to amuch shorter chain length and are therefore removed to a large extent inthe subsequent Kraft cook by a variety of hemicellulose hydrolysisreactions or by dissolution. The disadvantage of conventional high alphapulps used for lyocell is the resulting loss of yield by having toeliminate hemicelluloses.

[0011] In view of the expense of producing commercial high alpha pulps,it would be desirable to have alternatives to conventional high alphapulps for making lyocell products. In addition, manufacturers would liketo minimize the capital investment necessary to produce such types ofpulps by utilizing existing capital plants. Thus, there is a need forrelatively inexpensive, low alpha (e.g., high yield, high hemicellulose)pulps that have attributes that render them useful in lyocell-moldedbody production.

[0012] In U.S. Pat. No. 6,210,801, fully incorporated herein byreference in its entirety, assigned to the assignee of the presentapplication, low viscosity, high hemicellulose pulp is disclosed that isuseful for lyocell-molded body production. The pulp is made by reducingthe viscosity of the cellulose without substantially reducing thehemicellulose content. Such processes use an acid, or an acidsubstitute, or other methods therein described.

[0013] While the methods described in the '801 patent are effective atreducing the average degree of polymerization (D.P.) of cellulosewithout substantially decreasing the hemicellulose content, a furtherneed existed for a process that did not require a separate copper numberreducing step and which was readily adaptable to pulp mills that haveoxygen reactors, multiple alkaline stages and/or alkaline conditionssuitable for substantial D.P. reduction of bleached or semi-bleachedpulp. Environmental concerns have also generated a great interest inusing bleaching agents that reduce the use of chlorine compounds. Inrecent years, the use of oxygen as a delignifying agent has occurred ona commercial scale. Examples of equipment and apparatus useful forcarrying out an oxygen stage delignification process are described inU.S. Pat. Nos. 4,295,927; 4,295,925; 4,298,426; and 4,295,926. In U.S.Pat. No. 6,331,554, assigned to the assignee of the present application,fully incorporated herein by reference in their entirety, a highhemicellulose, low viscosity pulp is disclosed that is useful forlyocell-molded body formation. The pulp is made from an alkaline pulp bytreating the alkaline pulp with an oxidizing agent in a medium to highconsistency reactor to reduce the D.P. of the cellulose, withoutsubstantially reducing the hemicellulose or increasing the coppernumber.

[0014] Further efforts to reduce the cost of making lyocell-moldedbodies has resulted in U.S. patent application Ser. No. 09/842,274,fully incorporated by reference in its entirety. In the '274application, the assignee of the present invention describes pulps madefrom sawdust and other low fiber length wood using a procedure similarto that of the '554 patent. These pulps are high in hemicellulose andlow in viscosity, which makes them especially suitable forlyocell-molded body formation.

[0015] The forest industry continues to generate vast quantities ofbyproducts in the normal course of day-to-day forestry management andwood processing. These byproducts are for the most part underutilized.The need to conserve resources by utilizing wood byproducts in new wayspresents a unique opportunity. It would be advantageous to develop a lowcost pulp that is useful for making lyocell-molded bodies from all thisunderutilized wood, namely from the core wood or young or juvenile woodsuch as thinnings, hereafter referred to as low specific gravity wood.Thus, presenting a low cost alternative to the highly refined high-alphapulps.

SUMMARY OF THE INVENTION

[0016] One embodiment of the invention is a pulp having at least 7% byweight hemicellulose; a viscosity of less than or about 32 cP; a coppernumber less than or about 2; a weighted average fiber length less thanor about 2.7 mm; and a coarseness less than or about 23 mg/100 m. Inanother embodiment of the invention, a method for making lyocell-moldedbody is provided. The method includes dissolving a pulp in a solvent toform a cellulose solution; forming a lyocell-molded body from thesolution; and regenerating the molded body, wherein the pulp has atleast 7% by weight hemicellulose, a viscosity less than or about 32 cP;a copper number less than or about 2; a weighted average fiber lengthless than or about 2.7 mm; and a coarseness less than or about 23 mg/100m. The method can use a meltblowing, centrifugal spinning, spun bonding,or dry-jet wet technique.

[0017] In another embodiment of the invention, a method of making a pulpis provided. The method includes pulping of wet material with a specificgravity less than or about 0.41 using an alkaline pulping process; andbleaching the pulp to reduce the viscosity of the pulp to or about 32 cPor lower. The bleached pulp has at least 7% hemicellulose by weight, acopper number less than or about 2, a weighted average fiber length lessthan or about 2.7 mm, and a coarseness less than or about 23 mg/100 m.

[0018] In another embodiment of the invention, a lyocell product isprovided. The lyocell product has at least 7% hemicellulose by weight,and cellulose, wherein the pulp used to make the product has a viscosityless than or about 32 cP, a copper number less than or about 2, aweighted average fiber length less than or about 2.7 mm, and acoarseness less than or about 23 mg/100 m. Lyocell products can befibers, films, or non-woven webs, for example.

[0019] The use of low specific gravity wood can produce a lowerbrownstock viscosity for a given kappa number target. Using wood withlow specific gravity values reduce the bleach stage temperature and thechemical dose needed in the bleach plant to produce pulp havingacceptable lyocell specifications. Low specific gravity wood results invery low viscosity levels without increasing the copper number of thepulp or the concentration of carbonyl in the pulp above acceptablelevels. The process does not use an acid phase pretreatment prior topulping, and the subsequent bleaching conditions do not result in asubstantial decrease in hemicellulose content.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The foregoing aspects and many of the attendant advantages ofthis invention will become more readily appreciated as the same becomebetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0021]FIG. 1 is a flowsheet illustrating one embodiment of a method ofmaking a pulp according to the present invention; and

[0022]FIG. 2 is a flow sheet illustrating one embodiment of a method ofmaking a lyocell-molded body according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0023] Referring now to FIG. 1, a suitable method to produce a lyocelldissolving pulp from low specific gravity wood is illustrated. Themethod may be considered to include two broad processing areas, pulpingdepicted as block 126 and bleaching depicted as block 128.

[0024] In block 100, low specific gravity wood chips are loaded or fedinto a digester. Specific gravity, according to The Handbook of Pulpingand Papermaking, 2d ed., by Christopher J. Biermann, is the (unit less)ratio of the solid wood density to the density of water at the sametemperature. As used herein, specific gravity is the average specificgravity of any population of wood feedstock material. The solid wooddensity may be determined using the green volume, the oven-dry volume,or intermediate volumes. The wood chips used in practicing the inventioncan be made from any cellulose source. Contrary to conventionalthinking, low specific gravity wood has been found to be suitable foruse as a source of cellulose for making lyocell-molded bodies. Asuitable range of low specific gravity wood used for the presentinvention is any wood material having a specific gravity about equal orless than 0.41. Low specific gravity wood results in a lower brownstockpulp viscosity, which is believed to reduce the use of bleachingchemicals in the bleach plant. Representative sources of low specificgravity wood may be derived from “thinnings” and “juvenile” wood.Juvenile wood is defined as the first 10 growth rings surrounding thepith, according to Biermann. However, others define it as wood formednear the pith of the tree, often characterized by wide growth rings,lower density, and shorter fibers. However, in some instances thejuvenile wood may extend to the 15-ring or more. Specific gravityincreases with the increasing height of the tree, so specific gravity at16 feet, 32 feet, or 48 feet is incrementally greater than at the buttof the tree. In some embodiments, the specific gravity will be less than0.41, and could be less than 0.38, 0.36, 0.34, 0.32, or 0.30, or less.

[0025] Digesters for use in the present invention can include anydigester suitable to pulp low specific gravity wood. One example of asuitable digester is a continuous digester that is often referred to asa “Kamyr” digester. (It should be noted that Kamyr is the name of aCompany that designed and built such digesters and as such, the termKamyr is loosely associated with a continuous digester. Kamyr no longerexists as a Company. Such continuous digesters are supplied byKvaerner.) These digesters have been used in the pulp and paper industryfor several years with the first one being installed in Sweden in 1950.Over the years, the modifications have been made to these digesters toimprove their operation. The digester system may be either a singlevessel or a two-vessel system. “Kamyr” digesters are typically used inKraft or alkaline wood pulping, but may also be used in semi-chemicalpulping methods. Other continuous digesters, such as the M&D digesterand the Pandia digester, are also suitable to use in the presentinvention. However, the present invention can also be practiced usingany batch or other continuous digester.

[0026] Referring to FIG. 1, within the pulping process, block 126, thereare several operations, depicted as blocks 100-116. Loading, or feedingchips as discussed above, occurs in block 100. The wood chips may bepresteamed prior to cooking, block 102. Steam at atmospheric pressurepreheats the chips and drives off air so that liquor penetration will beenhanced. After the pre-steaming operation is completed, cooking liquor,referred to as white liquor, containing the pulping chemicals may beadded to the chips, block 104. The white liquor and chips are then fedinto the digester. In Kraft pulping, the active chemical compounds areNaOH and Na₂S. Other chemicals may be added to influence or impartdesirable effects on the pulping process. These additional chemicals arewell known to those of skill in the art. The present invention providesa lower brownstock pulp viscosity from relatively lower specific gravitywood as composed with wood having a higher specific gravity, i.e.,specific gravity is related to Kappa number.

[0027] Impregnation, block 106, is the period during which the chemicalsare allowed to impregnate the low specific gravity wood material. Goodliquor penetration helps assure a uniform cooking of the chips.

[0028] “Cooking” occurs in blocks 108 and 110. The co-current liquidcontact operation, block 108, is followed by the counter-current liquidcontact operation, block 110. Cooking of the low specific gravity woodoccurs during these two operations. In either block 108 or 110, thecooking liquor and chips can be brought to temperature.

[0029] Digester washing, block 112, is accomplished by introducing washliquor into the bottom of the digester and having it flowcounter-current to the cooked pulp. Cooking for the most part ends whenthe pulp encounters the cooler wash liquor.

[0030] Upon completion of the cook operation, and digester washing, thedigester contents are blown, block 112. Digester blowing involvesreleasing the wood chips and liquor at atmospheric pressure. The releaseoccurs with a sufficient amount of force to cause fiber separation. Ifdesired, the blow tank may be equipped with heat recovery equipment toreduce operating expenses.

[0031] In block 114, the pulp is sent from the blow tank to externalbrownstock pulp washers. The separation of black liquor from the pulpoccurs at the brownstock washers.

[0032] In one embodiment of a method of making a pulp from low specificgravity wood to be used in the manufacture of lyocell-molded bodies, thetime allowed for impregnation in block 106 is about 35 minutes. Theinitial percent effective alkali is about 8.5. The percent effectivealkali at 5 minutes is about 1.6. The percent sulfidity is about 29. Theliquor ratio is about 4. The initial temperature is about 110° C. Theresidual grams per liter of effective alkali is about 9.63. The residualpercent effective alkali is about 3.85. The pH is about 12.77, and the Hfactor is about 2.

[0033] In one embodiment of the co-current operation, block 108, thepercent effective alkali is about 4.2. According to Biermann, theeffective alkali is the ingredients that will actually produce alkaliunder pulping conditions. The percent sulfidity is about 29. Accordingto Biermann, the sulfidity is the ratio of sodium sulfide to the activealkali, expressed as a percent. The liquor addition time is about 1minute. The temperatures may be ramped to the final cooking temperaturewith a hold at one or more temperatures. The first temperature platformis about 154° C. The time to reach the temperature is about 9 minutesand the time at the temperature is about 5 minutes. A second and highercooking temperature at the co-current operation is provided at 170° C.The time to reach the second temperature is about 51 minutes and thetime at temperature is about 3 minutes. The effective alkali remainingafter a cook operation is called the residual alkali. The residual gramsper liter of effective alkali is about 9.42, following the co-currentoperation. The residual percent effective alkali is about 3.77. The pHis about 12.92, and the H factor is about 649.

[0034] In one embodiment of the counter-current operation, block 110,the percent effective alkali is about 8. The percent sulfidity is about29.2. Capability also exists for ramping to two different temperaturesin the counter-current step. However, in one embodiment, the first andsecond cooking temperatures are both about 171° C. The time to reachtemperature is about 54 minutes and the time at the temperature is about162 minutes. The effective alkali grams per liter is about 16.0. Thedisplacement rate is about 93 ml per minute. The displacement volume isabout 20 liters. The volumes given here are relatively small, since themethod was tested on a lab-scale bench reactor. However, with theparameters provided herein, and with no undue experimentation, theprocess can be scaled to any rate. The residual grams per liter ofeffective alkali is about 9.95. The residual percent effective alkali isabout 3.98. The pH is about 12.74 and the H factor is about 3877. In oneembodiment, the total time is about 319 minutes and the percenteffective alkali for the total cook is about 22.3.

[0035] In one embodiment, after washing, the viscosity of the brownstockpulp is about 153 cP. The total yield on oven dried wood is about 41.04.

[0036] Following the pulping process, generally depicted as referencenumeral 126 in FIG. 1, the brownstock pulp made from low specificgravity wood is bleached to reduce its viscosity. The bleaching processdoes not lead to a substantial reduction of the hemicellulose content ofthe pulp. The method according to the invention produces a bleacheddissolving pulp that is suitable for lyocell-molded body production.Bleaching of chemical pulps involves the removal of lignin with anattendant decrease in the pulp fiber length and viscosity. However, thebleaching process does not cause a substantial reduction to thehemicellulose content of the pulp. Bleaching brownstock pulp made fromlow specific gravity wood may require fewer chemicals than theconventional highly refined, high-alpha pulps presently being used forlyocell.

[0037] In one embodiment, the low specific gravity brownstock pulp madeaccording to the invention can be treated with various chemicals atdifferent stages in the bleach plant. The stages are carried out invessels or towers of conventional design. One representative bleachingsequence is ODEPD. The operations occurring in the bleaching plant arerepresented collectively by reference numeral 128 in FIG. 1. Otherembodiments of post bleaching the pulp after pulping are described inU.S. Pat. No. 6,331,354, and U.S. patent application Ser. No.09/842,274, incorporated herein by reference in their entirety.

[0038] The first stage of bleaching is an O stage, block 116. The Ostage comprises bleaching with oxygen. However, according to Biermann,some consider oxygen bleaching to be an extension of the pulpingprocess. Oxygen bleaching is the delignification of pulps using oxygenunder pressure. The oxygen is considered to be less specific for theremoval of lignin than the chlorine compounds. Oxygen bleaching takesplace in an oxygen reactor. Suitable oxygen reactors capable of carryingout the method of the present invention are described in U.S. Pat. Nos.4,295,925; 4,295,926; 4,298,426; and 4,295,927, fully incorporatedherein by reference in their entirety. The reactor can operate at a highconsistency, wherein the consistency of the feedstream to the reactor isgreater than 20% or it can operate at medium consistency, where themedium consistency ranges between 8% up to 20%. Preferably, if a highconsistency oxygen reactor is used, the oxygen pressure can reach themaximum pressure rating for the reactor, but more preferably is greaterthan 0 to about 85 psig. In medium consistency reactors, the oxygen canbe present in an amount ranging from greater than 0 to about 100 poundsper ton of the pulp, but is more preferably about 50 to about 80 poundsper ton of pulp. The temperature of the O stage ranges from about 100°C. to about 140° C.

[0039] In one embodiment of the method to make a pulp suitable to beused in making lyocell-molded bodies, a D stage, block 118 follows the Ostage, block 116. The D stage comprises bleaching the pulp coming fromthe oxygen reactor with chlorine dioxide. Chlorine dioxide is moreselective than oxygen for removing lignin. The amount of chlorinedioxide used in this stage ranges from about 20 to about 30 lb/ton,which may be lower than a conventional bleach plant that processes pulpfrom wood chips with a specific gravity not within the low specificgravity range of this invention. The temperature of the D stage rangesfrom about 50° C. to about 85° C.

[0040] In one embodiment of the method to make a pulp suitable to beused in making lyocell-molded bodies, an Ep stage, block 120, followsthe D stage, block 118. The Ep stage is the hydrogen peroxide reinforcedextraction stage where lignin is removed from the pulp using caustic inan amount ranging from about 20 to about 50 lb/ton. The amount ofhydrogen peroxide ranges from about 20 to about 60 lb/ton, which may belower than a conventional bleach plant that processes pulp from woodchips having a specific gravity not considered within the low specificgravity range of this invention. The temperature of the Ep stage rangesfrom about 75 to about 95° C.

[0041] In one embodiment, a second D stage, block 122, follows the Epstage, block 120. The amount of chlorine dioxide used in this stageranges from 10 to about 30 lb/ton, which may be lower than aconventional bleach plant that processes pulp from wood chips having aconventional specific gravity not considered to be within the lowspecific gravity range of this invention. The temperature of the D stageranges from about 60° C. to about 90° C.

[0042] One embodiment of a pulp made from low specific gravity wood hasa hemicellulose content of at least 7% hemicellulose, a pulp viscosityless than 32 cP, a copper number less than 2.0, and in some instancesless than 1.3 (TAPPI T430), a weighted average fiber length less than2.7 mm, and a coarseness less than 23 mg/100 m. Other embodiments ofpulps made according to the present invention have a combined copper,manganese, and iron content less than 2 ppm, a total metal load lessthan 300 ppm, and a silicon content less than 50 ppm. Lyocell moldedbodies made from the pulps of the invention will have a correspondinglyhigh hemicellulose content of at least 7% by weight, and cellulose.

[0043] Hemicellulose is measured by a sugar content assay based on TAPPIstandard T249 hm-85.

[0044] Methods for measuring pulp viscosity are well known in the art,such as TAPPI T230. Copper number is a measure of the carboxyl contentof pulp. The copper number is an empirical test used to measure thereducing value of cellulose. The copper number is expressed in terms ofthe number of milligrams of metallic copper, which is reduced fromcupric hydroxide to cuprous oxide in an alkaline medium by a specifiedweight of cellulosic material. The degree to which the copper numberchanges during the bleaching operation is determined by comparing thecopper number of the brownstock pulp entering the bleaching plant andthe copper number of the bleached pulp after the bleaching plant. A lowcopper number is desirable because it is generally believed that a highcopper number causes cellulose and solvent degradation during and afterdissolution of the bleached pulp to form a dope.

[0045] The weighted average fiber length (WAFL) is suitably measured bya FQA machine, model No. LDA93-R9704, with software version 2.0, made bythe Optest Company of Hawkesbury, Ontario, Canada.

[0046] Coarseness is measured using Weyerhaeuser Standard Method WMW-FQA.

[0047] Transition metals are undesirable in pulp because they acceleratethe degradation of cellulose and NMMO in the lyocell process. Examplesof transition metals commonly found in bleached pulps include iron,copper, and manganese. Preferably, the combined metal content of thesethree metals in the pulps of the invention is less than about 20 ppm byWeyerhaeuser Test No. AM5-PULP-1/6010.

[0048] Additionally, pulps of the invention have a total metal load ofless than 300 ppm by Weyerhaeuser Test No. AM5-PULP-1/6010. The totalmetal load refers to the combined amount, expressed in units of partsper million (ppm), of nickel, chromium, manganese, iron and copper.

[0049] Once the pulp has been bleached to reduce its viscosity withoutsubstantially increasing its copper number or decreasing thehemicellulose content, the pulp can either be washed in water andtransferred to a bath of organic solvent, such asN-methyl-morpholine-N-oxide (NMMO), for dissolution prior tolyocell-molded body formation. Alternatively, the bleached washed pulpcan be dried and broken into fragments for storage and/or shipping in aroll, sheet or bale, for example.

[0050] In order to make lyocell products from the low specific gravitywood pulps, the pulp is first dissolved in an amine oxide, preferably atertiary amine oxide. Representative examples of amine oxide solventsuseful in the practice of the present invention are set forth in U.S.Pat. No. 5,409,532, incorporated herein by reference in its entirety.The preferred amine oxide solvent is NMMO. Other representative examplesof solvents useful in the practice of the present invention includedimethylsulfoxide (D.M.S.O.), dimethylacetamide (D.M.A.C.),dimethylformamide (D.M.F.) and caprolactan derivatives. The bleachedpulp is dissolved in amine oxide solvent by any known means such as onesset forth in U.S. Pat. Nos. 5,534,113; 5,330,567; and 4,246,221,incorporated herein by reference in their entirety. The pulp solution iscalled dope. The dope is used to manufacture lyocell fibers, films, andnonwovens or other products, by a variety of techniques, including meltblowing, spun-bonding, centrifugal spinning, dryjet wet, or any othersuitable method. Examples of some of these techniques are described inU.S. Pat. Nos. 6,235,392; 6,306,334; 6,210,802; and 6,331,354,incorporated herein by reference in their entirety. Examples oftechniques for making films are set forth in U.S. Pat. Nos. 5,401,447;and 5,277,857, incorporated herein by reference in their entirety.Meltblowing, centrifugal spinning and spunbonding used to make lyocellfibers and nonwoven webs are described in U.S. Pat. Nos. 6,235,392 and6,306,334, incorporated herein by reference in their entirety. Dryjetwet techniques are more fully described in U.S. Pat. Nos. 6,235,392;6,306,334; 6,210,802; 6,331,354; and 4,142,913; 4,144,080; 4,211,574;4,246,221; incorporated herein by reference in their entirety.

[0051] One embodiment of a method for making lyocell products, includingfibers, films, and nonwoven webs from dope derived from pulp isprovided, wherein the pulp is made from low specific gravity wood, thepulp having at least 7% hemicellulose, a viscosity less than or about 32cP, a copper number less than or about 2, a weighted average fiberlength less than or about 2.7 mm, and a coarseness less than or about 23mg/100 m. The method involves extruding the dope through a die to form aplurality of filaments, washing the filaments to remove the solvent,regenerating the filaments with a nonsolvent, including water oralcohol, and drying the filaments.

[0052]FIG. 2 shows a block diagram of one embodiment of a method forforming lyocell fibers from the pulps made from low specific gravitywood according to the present invention. Starting with low specificgravity wood pulp in block 200, the pulp is physically broken down, forexample by a shredder in block 202. The pulp is dissolved with an amineoxide-water mixture to form a dope, block 204. The pulp can be wettedwith a nonsolvent mixture of about 40% NMMO and 60% water. The mixturecan be mixed in a double arm sigma blade mixer and sufficient waterdistilled off to leave about 12-14% based on NMMO so that a cellulosesolution is formed, block 208. Alternatively, NMMO of appropriate watercontent may be used initially to eliminate the need for the vacuumdistillation block 208. This is a convenient way to prepare spinningdopes in the laboratory where commercially available NMMO of about40-60% concentration can be mixed with laboratory reagent NMMO havingonly about 3% water to produce a cellulose solvent having 7-15% water.Moisture normally present in the pulp should be accounted for inadjusting the water present in the solvent. Reference is made toarticles by Chanzy, H., and A. Peguy, Journal of Polymer Science,Polymer Physics Ed. 18:1137-1144 (1980), and Navard, P., and J. M.Haudin, British Polymer Journal, p. 174 (December 1980) for laboratorypreparation of cellulose dopes in NMMO and water solvents.

[0053] The dissolved, bleached pulp (now called the dope) is forcedthrough extrusion orifices in a process called spinning, block 210, toproduce cellulose filaments that are then regenerated with anon-solvent, block 202. Spinning to form lyocell-molded bodies,including fibers, films, and nonwovens, may involve meltblowing,centrifugal spinning, spun bonding, and dry-jet wet techniques. Finally,the lyocell filaments or fibers are washed, block 214.

[0054] The solvent can either be disposed of or reused. Due to its highcosts, it is generally undesirable to dispose of the solvent.Regeneration of the solvent suffers from the drawback that theregeneration process involves dangerous, potentially explosiveconditions.

[0055] The following examples merely illustrate the best mode nowcontemplated for practicing the invention, but should not be construedto limit the invention.

EXAMPLE 1

[0056] A commercial continuous extended delignification process wassimulated in the laboratory utilizing a specially built reactor vesselwith associated auxiliary equipment, including circulating pumps,accumulators, and direct heat exchangers, etc. Reactor temperatures werecontrolled by indirect heating and continuous circulation of cookingliquor. The reactor vessel was charged with a standard quantity ofequivalent moisture free wood. An optional atmospheric pre-steaming stepmay be carried out prior to cooking. A quantity of cooking liquor,ranging from about 50% to 80% of the total, was then charged to thedigester along with dilution water to achieve the target liquor to woodratio. The reactor was then brought to impregnation temperature andpressure and allowed to remain for the target time. Following theimpregnation period, an additional portion of the total cooking liquorwas added to the reactor vessel, ranging from about 5% to 15% of thetotal. The reactor was then brought to cooking temperature and allowedto remain there for the target time period to simulate the co-currentportion of the cook.

[0057] Following the co-current portion of the cook, the remainder ofthe cooking liquor was added to the reactor vessel at a fixed rate. Therate is dependent on the target time period and proportion of cookingliquor used for this step of the cook. The reactor was controlled at atarget cooking temperature and allowed to remain there during thesimulation of the counter-current portion of the cook. Spent cookingliquor was withdrawn from the reactor into an external collectioncontainer at the same fixed rate. At the end of the cook, the reactorvessel was slowly depressurized and allowed to cool below the flashpoint. The reactor vessel was opened and the cooked wood chips werecollected, drained of liquor, washed, screened and made ready fortesting. Three cooks of low specific gravity wood chips were prepared,along with three cooks of non-low specific gravity wood.

EXAMPLE 2 Pulping Process Parameters for Low Specific Gravity Wood

[0058] One cook for low specific gravity wood chips had the followingparameters. TABLE 1 Wood Chip S.G. 0.410 Pre-Steam @ 110 C., minutes 5Impregnation: Time, minutes 35 % Effective Alkali, initial 8.5 % EA,second @ 5 minutes 1.6 % sulfidity 29 Liquor ratio 4 Temperature -degrees C. 110 Residual, G/L EA 9.63 Residual, % EA 3.85 PH 12.77H-factor 2 Pressure Relief Time, Minutes 3 Co-Current: % EffectiveAlkali 4.2 % sulfidity 29 liquor addition time, minutes 1 temperature -degrees C. 154 time to, minutes 9 time at, minutes 5 temperature -degrees C. 170 time to, minutes 51 time at, minutes 3 residual, G/L EA9.42 residual, % EA 3.77 PH 12.92 H-Factor 649 Counter-Current: %effective alkali 8 % sulfidity 29.2 temperature - degrees C. 171 timeto, minutes 54 time at, minutes 0 temperature - degrees C. 171 time to,minutes 0 time at, minutes 162 EA, G/L - strength 16.0 displacementrate, CC/M 93 displacement volume, liters 20.00 residual, G/L EA 9.95residual, % EA 3.98 PH 12.74 H-factor 3877 Total Time, Minutes 319 %Effective Alkali - Total Cook 22.3 Accepts, % on O.D. Wood 41.01Rejects, % on O.D. Wood 0.03 Total Yield, % on O.D. Wood 41.04 KappaNumber, 10 Minutes 16.80

EXAMPLE 3 Bleaching Process for Low Specific Gravity Wood

[0059] The pulp made by the process of Example 2 was bleached accordingto the following procedure.

[0060] O Stage

[0061] Inwoods low specific gravity wood chips were pulped into analkaline Kraft pulp with a kappa number of 16.8 (TAPPI Standard T236cm-85 and a viscosity of 239 cP (TAPPI T230). The brownstock pulp wastreated with oxygen in a pressure vessel with high consistency mixingcapabilities. The vessel was preheated to about 120° C. An amount ofsodium hydroxide (NaOH) equivalent to 100 pounds per ton of pulp wasadded to the alkaline pulp. The reaction vessel was then closed and thepressure was increased to 60 psig by introducing oxygen into thepressure vessel. Water was present in the vessel in an amount sufficientto provide a 10% consistency.

[0062] After 45 minutes, the stirring was stopped and the pulp wasremoved from the pressure vessel and washed. The resulting washed pulpviscosity was 35.3 cP, and had a kappa number of 3.8.

[0063] D Stage

[0064] The D stage treated the pulp processed in the O stage by washingit three times with distilled water, pin fluffing the pulp, and thentransferring the pulp to a polypropylene bag. The consistency of thepulp in the polypropylene bag was adjusted to 10% with the addition ofwater. Chlorine dioxide corresponding to an amount equivalent to 28.4pounds per ton of pulp was introduced to the diluted pulp by dissolvingthe chlorine dioxide in the water used to adjust the consistency of thepulp in the bag. The bag was sealed and mixed and then held at 75° C.for 30 minutes in a water bath. The pulp was removed and washed withdeionized water.

[0065] E_(p) Stage

[0066] The washed pulp from the D stage was then placed in a freshpolypropylene bag and caustic was introduced with one-half of the amountof water necessary to provide a consistency of 10%. Hydrogen peroxidewas mixed with the other one-half of the dilution water and added to thebag. The hydrogen peroxide charge was equivalent to 40 pounds per ton ofpulp. The bag was sealed and mixed and held for 55 minutes at 88° C. ina water bath. After removing the pulp from the bag and washing it withwater, the mat was filtered and then placed back into the polypropylenebag and broken up by hand.

[0067] D Stage

[0068] Chlorine dioxide was introduced a second time to the pulp in anamount equivalent to 19 pounds per ton of pulp with the dilution waternecessary to provide a consistency of 10%. The bag was sealed and mixed,and then held for 3 hours at 88° C. in a water bath. The treated pulphad a copper number of about 0.9 measured by TAPPI Standard T430 and hada hemicellulose (xylan and mannan) content of 12.7%.

EXAMPLE 4

[0069] Low specific gravity wood having a specific gravity of 0.410 waspulped using the Kraft process, and subsequently, bleached and treatedwith varying amounts of oxygen to reduce its viscosity. Components inthe pulps made using Inwoods low specific gravity wood chips are 7.2%xylans and 5.5% mannans.

[0070] Table 2 shows the results for three different cooking conditions.While brownstock pulp WAFL is provided, it is apparent that bleachingthe brownstock pulp to reduce its viscosity without substantiallyreducing the hemicellulose content, in accordance with the conditions ofthe present invention, will not result in any appreciable increase inthe bleached pulp WAFL and may in fact be lower than the brownstock pulpWAFL. TABLE 2 Inwoods Inwoods Inwoods chips chips chips Cook A Cook BCook C Chips Specific Gravity 0.410 0.410 0.410 Kappa of Brownstock 24.420.1 16.8 Yield % 43.2 41.4 41.0 Brownstock pulp viscosity 414 235 153(cP) Falling Ball Brownstock pulp WAFL 2.70 2.70 2.69 (mm) Brownstockpulp Coarseness 18.3 17.9 17.6 (mg/100 m) O2 pulp viscosity cP 55 34 28(100 lbs/ton NaOH) 7.6 6.0 3.8 kappa kappa kappa O2 pulp viscosity cP 8063 49 (60 lbs/ton NaOH) 6.0 7.5 5.6 kappa kappa kappa Bleached pulpcoarseness 32.4 21.8 (mg/100 m) Bleached pulp fibers/g × 10⁶ 4.8 4.6Bleached pulp viscosity (cP) 31.8 29.5 Bleached pulp intrinsic 4.1 4.2viscosity Bleached pulp 0.6 <0.6 Cu (ppm) Bleached pulp 12 14.3 Fe (ppm)Bleached pulp 1.5 3.6 Mn (ppm) Bleached pulp <0.4 <0.3 Cr (ppm) Bleachedpulp 41 31 Si (ppm)

COMPARATIVE EXAMPLE 5 Pulping Process Parameters for Non-Low SpecificGravity Wood

[0071] A conventional Tolleson wood chip made from wood having specificgravity of 0.495 was pulped using a Kraft process and subsequentlytreated with varying amounts of 5 oxygen to reduce its viscosity. Table3 shows the pulping conditions for one cook of Tolleson wood chips.TABLE 3 Wood Chips S.G. 0.495 Pre-Steam @ 110 C., minutes 5Impregnation: time, minutes 35 % Effective Alkali, initial 8.5 % EA,second @ 5 minutes 1.6 % sulfidity 30.5 liquor ratio 4 temperature -degrees C. 110 residual, G/L EA 9.17 residual, % EA 3.67 PH 13.24H-factor 2 Pressure Relief Time, Minutes 2 Co-Current: % EffectiveAlkali 4.2 % sulfidity 30.5 liquor addition time, minutes 1temperature - degrees C. 157 time to, minutes 14 time at, minutes 0temperature - degrees C. 170 time to, minutes 54 time at, minutes 0residual, G/L EA 8.31 residual, % EA 3.32 PH 13.07 H-Factor 680Counter-Current: % Effective Alkali 8 % sulfidity 30.0 Temperature -degrees C. 171 Time to, minutes 54 Time at, minutes 0 Temperature -degrees C. 171 Time to, minutes 0 Time at, minutes 162 EA, G/L -strength 20.4 Displacement rate, CC/M 73 Displacement volume, liters15.87 Residual, G/L EA 9.72 residual, % EA 3.89 PH 13.18 H-factor 3975Total Time, Minutes 319 % Effective Alkali - Total Cook 22.3 Accepts, %on O.D. Wood 44.23 Rejects, % on O.D. Wood 0.13 Total Yield, % on O.D.Wood 44.36 Kappa Number, 10 Minutes 17.75

[0072] Table 4 shows the results of three different cooks using. aconventional Tolleson wood chip made from a non-low specific gravitywood. Components in the pulps made using Tolleson non-low specificgravity wood chips are 6.5% xylose; 6.6% mannose; 5.7% xylans; and 5.9%mannans. TABLE 4 Tolleson Tolleson Tolleson chips chips chips Cook ACook B Cook C Chips Specific Gravity 0.495 0.495 0.495 Kappa ofBrownstock 26.9 20.8 17.8 Yield % 46.6 46.1 44.4 Brownstock pulp 633 358243 viscosity (cP) Falling Ball Brownstock pulp WAFL 4.13 4.14 4.19 (mm)Brownstock pulp 26.1 24.4 24.3 Coarseness (mg/100 m) O2 pulp viscositycP 96 43 41 (100 lbs/ton NaOH) 6.4 6.9 4.7 kappa kappa kappa O2 pulpviscosity cP 180 88 70 (60 lbs/ton NaOH) 8.3 5.5 6.2 kappa kappa kappaBleached pulp coarseness 24.9 27.5 (mg/100 m) Bleached pulp 3.8 2.8fibers/g × 10⁶ Bleached pulp viscosity 28.5 24.2 (cP) Bleached pulpintrinsic 4.3 4 viscosity Bleached pulp Cu (ppm) <0.6 <0.7 Bleached pulpFe (ppm) 11.5 16 Bleached pulp Mn (ppm) 5 6 Bleached pulp Cr (ppm) <0.40.3 Bleached pulp Si (ppm) ≧1 32

[0073] It can be seen that the viscosity of the pulps made from theInwoods low specific gravity wood chips is lower than the viscosity ofthe pulps made from the Tolleson non-low specific gravity wood chips.

[0074] It can be seen that the viscosity of the pulps made from theInwoods low specific gravity wood chips is lower than the viscosity ofthe pulps made from the Tolleson non-low specific gravity wood chips.

[0075] While the preferred embodiment of the invention has beenillustrated and described, it will be appreciated that various changescan be made therein without departing from the spirit and scope of theinvention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A method for making alyocell product, comprising: dissolving a pulp in a solvent to form acellulose solution; forming a lyocell product from the solution; andregenerating the product, wherein the pulp comprises at least 7% byweight hemicellulose, a viscosity less than 32 cP, a copper number lessthan 2, a weighted average fiber length less than 2.7 mm, and acoarseness less than 23 mg/100 m.
 2. The method of claim 2, wherein themethod is one of meltblowing, centrifugal spinning, spun bonding, anddryjet wet.